We present new measurements of radiative lifetimes for six energy levels of
singly ionized uranium, U II, using laser-induced fluorescence technique. One of the levels,
5f36d7p6M13/2 at 26191 cm-1, decays by a transition at 3859.6 Å. This
line has recently been observed in the spectrum of
the metal-poor star CS1082-001, the first detection of uranium outside the solar system.
The λ3859 line can be used as the presently most accurate
cosmochronometer (Cayrel et al. [CITE]). Our value of the lifetime of the 6M13/2
level is ns, and it confirms
the f-value used in the Nature article by Cayrel et al. ([CITE]), which is based on an
experimental lifetime of ns (Chen & Borzileri [CITE]). The new measurement
also removes the doubt about the choice between that value and other f-values in the
literature, differing by a factor of 3. Adopting the
same branching fraction as Chen & Borzileri ([CITE]) for the 3859.6 Åline, we derive a
gf-value of 0.68, which is 8% higher than the value used by Cayrel et al. ([CITE]).
Of significance for the chronometer is also the reduced uncertainty of the radiative
lifetime, 4% compared to 25% , and consequently of the f-value, which should decrease
the uncertainty in the determination of the stellar age considerably.

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